This invention relates generally to the field of integrated circuits. More specifically, the invention relates to a method and apparatus for indicating an over-current condition in a switching circuit.
FIG. 1 depicts, at a high level, a system 10 known to the prior art for indicating an over-current condition in a switching circuit 14. Such systems are disclosed, for example, in U.S. Pat. No. 5,903,422 to Hosokawa and U.S. Pat. No. 6,108,182 to Pullen. The switching circuit 14 shown is a DC to DC buck converter that maintains a predefined voltage level across the load by switching current through an inductor 15. The switching of current is done using a switch 16, which can be for example a P-channel or N-channel field effect transistor (xe2x80x9cFETxe2x80x9d) device. The system 10 for indicating an over-current condition includes an offset voltage source 18, a low-pass filter 22, a comparator 26, and a logic element 30. The comparator 26 compares the voltages applied to its positive and negative terminals and generates a voltage difference representing the voltage drop across the switch 16 in the switching circuit 14. The offset voltage (VOFFSET) generated by source 18 is added to the input voltage (VIN) to set the voltage level at which the comparator 26 output signal (COMOUT) transitions from a low state to a high state. The low-pass filter 22 across the input terminals of the comparator 26 filters out high-frequency switching noise to avoid false indications of over-current. A problem with the use of the low-pass filter 22 is that the over-current system 10 is unusable during small xe2x80x9cONxe2x80x9d times of the switch 16. Activating the switch 16 for short times is desirable in switch mode power converters in order to keep external component sizes small. The system 10 uses the logic element 30 to ensure that any indication of an over-current condition is made only when the switch 16 is in a closed position (i.e., xe2x80x9cONxe2x80x9d). A problem with this approach is that all the circuitry from the switching circuit 14 to the logic element 30 must process all transients and noise conditions. The present invention addresses the disadvantages of the above techniques.
It is therefore an object of the invention to detect over-current conditions for pulses with narrow xe2x80x9cONxe2x80x9d times and to filter out noise effectively. This ability eliminates the need for input filtering of the over-current detection input terminals and allows for high switching speeds and smaller external components. The technique protects against false triggering caused by a transient load condition or supply line noise. The technique combines the advantages of pulse by pulse over-current detection with the noise immunity of an average over-current detection. For example, ten 1 xcexcs pulses are equivalent to twenty 500 ns pulses or one 10 xcexcs pulse of similar magnitude, however, depending on the period, each scenario requires a different fault duration to trigger an indication. The fault indication effectively indicates the average power in the switch. In one embodiment the CMOS trip threshold is dependent on the supply voltage, the over-current technique is immune to false triggering due to changes in line voltage.
In one aspect the invention relates to a method to indicate an over-current condition in a switching circuit. The method includes monitoring a monitor voltage from the switching circuit, charging an energy storage device in response to the monitor voltage and a reference voltage, and generating an indication signal in response to the charging of the storage device. In another embodiment, the method further includes charging the energy storage device at a charge rate in response to the monitor voltage and a reference voltage, and discharging the energy storage device at a discharge rate. In another embodiment, the discharge rate is less than the charge rate.
In another embodiment, the method further includes receiving an enable signal, wherein the charging step includes charging the energy storage device in response to the monitor voltage, the reference voltage and the enable signal. In another embodiment, the method further includes generating the enable signal when a switching device within the switching circuit is in a closed state. In another embodiment, the method further includes determining the monitor voltage in response to a voltage drop across a switching device in the switching circuit. In another embodiment, the method further includes generating an indication signal in response a storage voltage of the energy storage device exceeding a second reference voltage. In another embodiment, the method further includes controlling the switching circuit in response to the indication signal. In another embodiment, the switching circuit is a synchronous, DC to DC converter.
In another aspect, the invention relates to a system to indicate an over-current condition in a switching circuit. The system includes a control module, an energy storage module and an indicator module. The control module has a first terminal configured to receive a monitor voltage from the switching circuit, and a second terminal, wherein the control module generates at the second terminal a control signal in response to the monitor voltage and a first reference voltage. The energy storage module has a first terminal in communication with the second terminal of the control module, a second terminal, and an energy storage device in communication with the second terminal of the energy storage module, wherein the energy storage device is charged in response to the control signal, thereby generating a storage voltage at the second terminal. The indicator module has a first terminal in communication with the second terminal of the energy storage module, and a second terminal, wherein the indicator module generates at the second terminal an indication signal in response to the charge signal.
In one embodiment, the control module further includes a third terminal configured to receive an enable signal, wherein the control module generates at the second terminal a control signal in response to the monitor voltage, the first reference voltage and the enable signal. In another embodiment the monitor voltage is a first monitor voltage and the control module further includes a third terminal, an amplifier and a comparator. The third terminal is configured to receive a second monitor voltage. The amplifier includes a first terminal in communication with the first terminal of the control module, a second terminal in communication with the third terminal of the control module, and a third terminal. The comparator includes a first terminal in communication with the third terminal of the amplifier, a second terminal in communication with the second terminal of the control module, and a third terminal in communication with the third terminal of the control module. In another embodiment, the amplifier further includes a fourth terminal configured to receive an enable signal.
In another embodiment, the energy storage module further includes a first current source, a switch and a second current source. The first current source includes a first terminal in communication with the second terminal of the energy storage module, and a second terminal. The switch includes a first terminal in communication with the second terminal of the first current source, a second terminal in communication with the first terminal of the energy storage module, and a third terminal in communication with the second terminal of the energy storage module. The second current source includes a first terminal in communication with the second terminal of the energy storage module, and a second terminal. In another embodiment, the first current source is configured to flow current at a first rate and the second current source is configured to flow current at a second rate, the second rate being less than the first rate. In another embodiment, the energy storage device is a capacitor.